The present invention is directed to high refractive index, optically clear and soft copolymers, and ophthalmic lenses formed therefrom. More particularly, the invention relates to a copolymer having as a first constituent a hydrophobic acrylamide monomer.
The physiology of the human eye includes an anterior chamber located between the cornea, or outer surface of the clear part of the eye, and the iris, the pigmented portion of the eye that is responsive to light, and a posterior chamber, filled with vitreous humor. A crystalline lens, which includes a lens matrix contained within a capsular bag, is located behind the iris and separates the iris from the posterior chamber. The crystalline lens is attached to the ciliary muscle by cord-like structures called zonules. Lining the rear of the posterior chamber is the retina, the light sensing organ of the eye, that is an extension of the optic nerve. In young, healthy eyes, contraction and relaxation of the ciliary muscle shapes the natural crystalline lens to the appropriate optical configuration for focusing light rays entering the eye on the retina.
As the natural crystalline lens ages, however, the structure of the lens matrix of the crystalline lens changes, becoming hazy and relatively inflexible. Eventually, the hazing of the lens matrix may progress to the point where the lens is considered cataractous, which may seriously occlude the amount of light passing through the crystalline lens and ultimately onto the retina. Fortunately, modern surgical techniques have been developed which allow removal of the cataractous lens matrix so that light may once again pass unimpeded onto the retina.
Presently, a cataractous crystalline lens matrix is removed from an eye using a procedure whereby the cataractous natural lens matrix is extracted from the capsular bag of the lens through an anterior capsulotomy. Typically, the cataractous lens matrix is removed from the capsular bag through the anterior capsulotomy using phaco-emulsification and aspiration. Alternatively, the cataractous lens matrix may be removed using several other well known techniques whereby the cataractous material is broken up and aspirated from the capsular bag. After extraction of the cataractous lens matrix, an intraocular lens may be implanted within the remaining capsular bag. However, while the procedure to remove the emulsified natural lens can be accomplished with about a three millimeter incision in the cornea, about a six millimeter incision is required to accommodate the full diameter of the intraocular lens to be implanted.
In order to reduce the size of the incision required for implantation of an intraocular lens, and thus limit the trauma to the eye, intraocular lenses made of relatively soft material that can be rolled, folded or otherwise deformed for insertion into the eye were developed, replacing conventional intraocular lenses made of relatively hard material, such as polymethylmethacrylate (PMMA). Soft intraocular lenses have typically been made from hydrogel, silicone, or acrylic material. Intraocular lenses made from hydrogel and silicone typically have a relatively low refractive index, requiring a thicker lens. Further, while silicone lens material has a low glass transition temperature (lower than xe2x88x92100xc2x0 C.), it unfolds quickly in the eye, risking damage to eye tissue. Acrylic material has been found to have a relatively high refractive index and unfold in a slower amount of time. However, most acrylic lens materials have relatively high glass transition temperatures at about room temperature. Therefore, further improvements in the properties of acrylic lens materials would be desirable.
It would be desirable to provide an acrylic material with a higher refractive index than conventional silicone and hydrogel materials to permit the lens to be much thinner and thus have more controllable folding release. Also, it would be desirable to provide an acrylic material with a lower glass transition temperature than conventional acrylic materials such that it can be folded at lower temperatures.
What has been needed and heretofore unavailable, is an acrylic material having improved properties, including a high refractive index and low glass transition temperature, for use in intraocular lenses. The present invention satisfies these needs and others.
The present invention provides a soft hydrophobic acrylamide copolymer useful in the fabrication of ophthalmic lenses, including intraocular lenses and other implantable ocular devices, such as intraocular contact lenses. The hydrophobic acrylamide copolymer has a water content of less than about 1 percent by weight.
In one embodiment, the soft hydrophobic acrylamide copolymer comprises a first constituent that is a hydrophobic acrylamide monomer. The hydrophobic acrylamide monomer may be N-benzyl-N-isopropylacrylamide (BPA) which may be present in an amount of about 5 to about 50 percent by weight of the copolymer, and more particularly, about 12 to about 25 percent by weight of the copolymer.
In another embodiment, the copolymer also includes a second constituent that forms copolymers having a glass transition temperature (Tg) of less than about 22xc2x0 C. This constituent may consist of alkyl acrylate or methacrylate monomers or mixtures thereof. In one embodiment, the second constituent is n-butyl acrylate. In another embodiment, it is a mixture of n-butyl acrylate and n-butyl methacrylate. In yet another embodiment, the second constituent is a mixture of n-butyl acrylate and ethyl methacrylate.
In still another embodiment, the copolymer includes a crosslinking agent, such as ethylene glycol diacrylate, ethylene glycol dimethacrylate, 1,3-butanediol diacrylate, 1,3-butanediol dimethacrylate, 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, poly(ethylene glycol)diacrylate, poly(ethylene glycol)dimethacrylate. In one embodiment, ethylene glycol dimethacrylate is used as the crosslinking agent.
In yet another embodiment, an ultraviolet (UV) light absorbing constituent, such as acrylate or methacrylate functionalized benzotriazoles and benzophenones, is also included in the copolymer. An initiating constituent, such as peroxide, peroxydicarbonate, azo free radical initiators, and UV initiators may also be included.
The copolymers of the present invention have a high refractive index, at least about 1.47 (at 20xc2x0 C.) in one embodiment. The glass transition temperature (Tg) of the copolymers is also low, being at most about 5xc2x0 C. in one embodiment.
In one embodiment, an ophthalmic lens is formed from the soft hydrophobic acrylamide copolymer. In particular, the ophthalmic lens may be an intraocular lens. The ophthalmic lens may also be an intraocular contact lens or other implantable ocular device.
The copolymers of the present invention are particularly suited for use in ophthalmic lenses due to their high refractive index and optical clarity. The copolymers are also soft materials with low glass transition temperatures, permitting lenses formed therefrom to be deformed for insertion at lower temperatures. The copolymers also have high strength and flexibility. They are also photostable and Yttrium Aluminum Garnet (YAG) laser stable.
Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the drawings, which illustrate, by way of example, various embodiments, principles and features of the invention.